Engine valves

ABSTRACT

The present invention includes an engine valve including a stem portion and a head portion disposed at one end of the stem portion. A heat insulation layer is formed on a surface of the head portion. A heat conductive layer is formed on a surface of the stem portion.

This application claims priority to Japanese patent application serial number 2008-256275, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to engine valves.

2. Description of the Related Art

In recent years, automobile engines have been increasingly altered by consumers to performance. However, as the engines are souped-up, combustion temperature of the engines becomes higher, which can cause potential damage or earlier deterioration of engine components. Therefore, it has been proposed to set the air-fuel ratio of a fuel mixture to be rich in fuel in order to lower the combustion temperature. However, when the fuel mixture that is rich in fuel is combusted, an HC component contained in an exhaust gas may increase to cause potential deviation from emission regulations. In particular, in recent years, emission regulations tend to become stricter from a viewpoint of environmental concerns. Therefore, it is desired to achieve souping-up of engines while a fuel mixture having a theoretical air-fuel ratio is combusted to meet emission regulations. However, if souping-up of engines is achieved with combustion of a fuel mixture having a theoretical air-fuel ratio, increase in combustion temperature is inevitable. Hence, improvements of engine components are necessary to be made.

Engine valves for controlling intake of a fuel mixture into a combustion chamber of an engine and for controlling discharge of an exhaust gas from the combustion chamber are examples of engine components that need the above improvements. The engine valves generally have a stem portion and a mushroom-like head portion disposed at one end of the stem portion. The heat may be transmitted to the engine valve from a front side of the head portion, which faces to the combustion chamber, and the heat may be dissipated from a face part contacting with a valve seat. The stem portion slidably contacts a valve guide. In the case of an intake valve, the heat may be also dissipated from a back side of the head portion by the intake air. However, in the case of an exhaust valve, the heat may be transmitted from the exhaust gas to a back side of the head portion, and therefore, the temperature of the exhaust valve tends to become higher than the temperature of the intake valve.

The balance between transmission and dissipation of heat described above governs the temperature of the engine valve during the operation of the engine valve. In the case of the exhaust valve, it is likely that the amount of dissipation of heat is smaller than the amount of transmission of heat to the valve. Therefore, depending on the operation condition of the engine, the head portion may have a high temperature and a heat load to the valve increases. For this reason and in view of the durability, martensitic or austenitic heat resisting steel having a good high-temperature property has been used in some known engine valves. According to the other examples, nickel alloy, aluminum alloy, magnesium alloy or titanium alloy is used for achieving lightweight construction. However, in general, heat resisting steels are relatively expensive and aluminum alloy or the like has a problem in heat resisting strength. For instance, the head portion of the engine valve may be heated to be more than 900° C. in some cases. Although nickel alloy may keep a good heat resistance strength until 850° C., it does not have a good heat resistance strength when the temperature is increased to 900° C. or more.

For the above reason, there has been proposed to reduce the temperature load to the engine valve by improving the structure of the engine valve itself. For example, Japanese Laid-Open Patent Publication No. 2007-32465 has proposed to construct the engine valve to have a hollow structure in order to mainly improve the dissipation of heat from the stem portion. Japanese Laid-Open Patent Publication Nos. 2003-307105 and 4-311611 have proposed to form a ceramic-type heat insulation layer on a surface of the head portion in order to reduce transmission of heat to the valve.

However, the manufacturing cost of the engine valve may be increased if the engine valve is constructed to have a hollow structure. In particular, in the case that this structure is applied to an exhaust valve, it may be necessary to fill a refrigerant, such as sodium, into the hollow space, and therefore, the material cost may be increased. In addition, if the amount of transmission of heat to the valve via the head portion is large, dissipation of heat may soon reach a limit. In the case that the heat insulation layer is formed on the surface of the head portion, it may be possible to reduce transmission of heat to the valve to some extent. However, the effect of reduction of transmission of heat is limited and it is not possible to improve dissipation of heat from the stem portion.

Therefore, there is a need in the art for engine valves that can reduce a heat load applied thereto without accompanying substantial increase in cost.

SUMMARY OF THE INVENTION

One aspect according to the present invention includes an engine valve having a stem portion and a head portion disposed at one end of the stem portion. A heat insulation layer is formed on a surface of the head portion. A heat conductive layer is formed on a surface of the stem portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of an engine valve according to an embodiment of the present invention;

FIG. 2 is a schematic structural view of a valve operating mechanism incorporating the engine valve shown in FIG. 1; and

FIG. 3 is an enlarged view of a part of FIG. 2 and showing the state where an exhaust port is opened by the engine valve.

DETAILED DESCRIPTION OF THE INVENTION

Each of the additional features and teachings disclosed above and below may be utilized separately or in conjunction with other features and teachings to provide improved engine valves. Representative examples of the present invention, which examples utilize many of these additional features and teachings both separately and in conjunction with one another, will now be described in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention.

Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Moreover, various features of the representative examples and the dependent claims may be combined in ways that are not specifically enumerated in order to provide additional useful embodiments of the present teachings.

In one embodiment, an engine valve includes a stem portion and a head portion disposed at one end of the stem portion. A heat insulation layer formed on a surface of the head portion. A heat conductive layer formed on a surface on the stem portion. The stem portion may have a rod-like configuration and may slidably contact a valve guide of a cylinder head of an engine. The head potion may have a face part that can contact a valve seat of the cylinder head. The head portion may also have a front side surface and a back side surface. The back side surface extends from the face part toward the stem portion and may be called as a neck part.

With this construction, transmission of heat to the head portion can be inhibited by the heat insulation layer. In addition, because the heat conductive layer is formed on the surface of the stem portion, the heat can be effectively dissipated from the stem portion. Therefore, a potential heat load applied to the engine valve can be effectively reduced. As a result, a broader choice of materials for the valve body is possible, and there is no need to configure the valve body to have a hollow construction.

Preferably, the heat conductive layer is made of aluminum nitride or chrome nitride. Aluminum nitride or chrome nitride is suitable as the material of the heat conductive layer, because these materials have a good heat resisting property in addition to a good heat conductivity.

Although the present invention can be applied to an engine valve having a hollow structure, the present invention is advantageously applied to an exhaust valve having a sold valve body in order to maximize the advantage of the present invention. Thus, the manufacturing cost and the material cost of a solid valve body is lower than a hollow valve body. In addition, a large heat load reduction effect can be achieved, because an exhaust valve may be heated to a higher temperature than an intake valve.

The heat insulation layer may be formed on one or both of a front side surface and a back side surface of the head portion. In the case that the engine valve is an exhaust valve, an exhaust gas flows along the back side surface of the engine valve when the exhaust gas is discharged from a combustion chamber of the engine into an exhaust port. Because a cross sectional area across the back side surface is smaller than a cross sectional area across the front side surface, a heat capacity at the back side surface is smaller than a heat capacity at the front side surface. In other words, a potential heat load at the back side surface is larger than that at the front side surface, because a heat capacity at the back side surface is smaller than that at the front side surface. Therefore, if the heat insulation layer is formed on one of the back side surface and the front side surface, it is preferable that the back side surface is preferentially selected. Although it is most preferable that the heat insulation layer is formed on both of the front side surface and the back side surface, it is still possible to achieve a sufficient heat insulation effect by forming the heat insulation layer only on the front side surface of the head portion. Thus, it is possible to inhibit transmission of heat directly from the combustion chamber of the engine by the heat insulation layer on the front side surface of the head portion.

Preferably, the heat insulation layer is made of ceramic-based material, so that it is possible to reliably inhibit transmission of heat to the engine valve.

An embodiment of the present invention will now be described with reference to FIGS. 1 to 3. Referring to FIG. 1, an engine valve 1 includes a rod-like stem portion 2 and a mushroom-like head portion 3 disposed at one end of the stem portion 2. The head portion 3 has a diameter that increases in a direction away from the stem portion 2. The head portion 3 has a face part 3 a for contacting with a valve seat 19 that will be explained later. In this embodiment, the engine valve 1 has a solid body although the engine valve 1 may have a hollow body. In addition, the engine valve 1 of this embodiment is suitably used for an exhaust valve that may be heated to a higher temperature than an intake valve, although it is possible to use the engine valve 1 as an intake valve.

Heat insulation layers 4 made of material having a good heat insulation property are formed on a front side surface 3 b of the head portion 3 facing a combustion chamber (not shown) of an engine 10 (see FIG. 2) and on a back side surface (neck surface) 3 c of the head portion 3 facing to an exhaust port 18 of the engine 10, respectively. No heat insulation layer is formed on a surface of the face portion 3 a. A heat conductive layer 5 made of material having a good heat conductivity is formed on a surface of the stem portion 2.

Various ceramic materials, each having a good heat resistance and a good heat insulation property, can be used as the material of the heat insulation layers 4. For example, oxide ceramics including alumina, cordierite, zirconia, zircon, titanium oxide and magnesia, carbide ceramics including silicon carbide, and nitride ceramics including silicon nitride can be used. It is also possible to use aluminum silicate, chrome oxide, WC—Co alloy, WC—Ni—W—Cr₃C₂ alloy and Cr₃C₂—Ni—Cr alloy. The thickness of the heat insulation layers 4 may be determined by taking into account a heat insulation effect and reduction in weight and may preferably be about 0.1 to 2 mm. A single layer or a plurality of stacked or laminated layers may constitute each of the heat insulation layers 4.

As the material of the heat conductive layer 5, aluminum nitride or chrome nitride each having a good heat conductivity and a good heat resisting property may preferably used. The thickness of the heat conductive layer 5 may be determined by taking into account of necessary heat dissipation property and reduction in weight and may preferably be about 1 to 100 μm.

There is no limitation to the material of the body of the engine valve 1 and any materials used in known engine valves can be used as the material of the body. However, if a natural oxide layer is formed on the surface of the body, such an oxide layer may interfere with the heat conductivity. Therefore, it may be preferable to remove such an oxide layer from the surface of the engine valve body before forming the heat conductive layer 5. Although it is possible to form the heat conductive layer 5 on the entire surface of the stem portion 2, the heat conductive layer 5 may be formed on at least a part of the surface of the stem portion 2, which slidably contacts with a valve guide 12 of the engine 10 (see FIG. 2). It is preferable to form the heat conductive layer 5 only on the part of the surface of the stem portion 2, which slidably contacts with the valve guide 12, from a viewpoint of reduction of material cost.

The heat insulation layers 4 and the heat conductive layer 5 may be formed using various techniques, such as gas burning, arc spraying, plasma spraying, explosion spraying, spattering and ion-plating techniques, etc.

A valve operating mechanism for the engine valve 1 will be generally described prior to explanation of the operation of the engine valve 1. It should be understood that the valve operating mechanism explained below is only an example and that the engine valve 1 can be applied to any other valve operating mechanisms having different configurations form that explained below.

Referring to FIG. 2, the stem portion 2 of the engine valve 1 (exhaust valve in this embodiment) is axially (vertically in FIG. 2) slidably inserted into the valve guide 12 that is fixedly attached to a cylinder head 11 of the engine 10. A spring retainer 15 is mounted to an upper end 2 a (an end opposite to the head portion 3) via a cotter 14 that is in engagement with a cotter receiving groove 13 formed in the upper end 2 a. A compression coil spring 16 is interposed between a spring seat 11 a and the spring retainer 15. The spring seat 11 a is formed on the upper surface of the cylinder head 11 in such a way that the spring seat 11 a surrounds the valve guide 12. A valve seat 19 is fixedly attached to the inner circumference of an opening of the exhaust port 18 on the side of the combustion chamber. The engine valve 1 is normally biased upwardly by the coil spring 16, so that the face portion 3 a of the head portion 3 contacts the valve seat 19 for closing the opening of the exhaust port 18. A cam 21 is mounted to a camshaft 20 that is rotatably driven by a crankshaft (not shown). A rocker arm 23 is swingably mounted to a rocker shaft 22 that extends parallel to the camshaft 20.

When a mixture of the fuel (e.g., gasoline) and the air is combusted within the combustion chamber, the camshaft 20 rotates for discharging an exhaust gas produced after combustion of the mixture. Then, the rocker arm 23 swings as the cam 21 rotates, so that the engine valve 1 is pressed downward against the biasing force of the coil spring 16. Therefore, the head portion 3 of the engine valve 1 moves to be separated from the valve seat 19, so that the exhaust port 18 is opened. As the rocker arm 23 returns to its original position, the engine valve 1 moves upward by the biasing force of the coil spring 16. Hence, the face part 3 a of the head portion 3 contacts the valve seat 19 to again close the exhaust port 19.

The operation of the engine valve (exhaust valve) 1 will be described in relation to the valve operating mechanism described above. When the mixture of the fuel and the air is combusted within the combustion chamber, heat is produced by combustion and is transmitted to the engine valve 1 via the front side surface 3 b of the head portion 3. However, because the heat insulation layer 4 is formed on the front side surface 3 b, transmission of heat from the combustion chamber to the engine valve 1 via the front side surface 3 b can be reduced. When the engine valve 1 moves to be separated from the valve seat 19 in order to discharge the exhaust gas from the combustion chamber to the exhaust port 18, the exhaust gas that has a high temperature is discharged into the exhaust port 18 in such a way that the exhaust gas flows along the back side surface 3 c. Therefore, the heat may be transmitted from the exhaust gas to the engine valve 1 also via the back side surface 3 c. However, because the heat insulation layer 4 is also formed on the back side surface 3 c, it is possible to reduce transmission of heat to the engine valve 1 via the back side surface 3 c. In this way, because transmission of heat to the head portion 3 can be reduced at both of the front side surface 3 b and the back side surface 3 c, the heat load applied to the head portion 3 including the back side surface 3 c at the neck portion having a small heat capacity can be reduced.

However, the transmission of heat to the head portion 3 is not completely stopped, therefore the head portion 3 temperature may further increase. As a result, the heat transmitted to the head portion 3 is conducted to the stem portion 2 and is thereafter dissipated to the valve guide 12 that slidably contacts the stem portion 2. Because the heat conductive layer 5 is formed on the surface of the stem portion 2, conduction of heat from the stem portion 2 to the valve guide 12 can be effectively performed. In other words, the heat conductive layer 5 improves the ability of heat to dissipate from the stem portion 2 to the valve guide 12.

Therefore, the heat load to the head portion 3 can be further reduced. When the engine valve 1 is closed to cause the face part 3 a to contact with the valve seat 19, the heat can be also dissipated from the face part 3 a to the valve seat 19. Because no heat insulation layer is formed on the face part 3 a, the heat dissipation ability is not lowered at this part.

Preferably, the valve guide 12 may be made of copper alloy that has a good heat conductively, so that the dissipation of heat from the stem portion 2 can be further improved. In general, the valve seat 19 is constructed as a separate member from the cylinder heat 11. In many cases, a joint surface between the cylinder head 11 and the valve seat 19 is not always a flat surface but has a roughness in micron units. Therefore, a clearance may be formed due the roughness to inhibit the heat conductive ability and to eventually lower the ability of dissipating heat from the engine valve 1. For this reason, the valve seat 19 is preferably configured as a clad seat that is formed integrally with the cylinder head 11 by building up the cylinder head 11. 

1. An engine valve comprising: a stem portion; a head portion disposed at one end of the stem portion; a heat insulation layer formed on a surface of the head portion; and a heat conductive layer formed on a surface on the stem portion.
 2. The engine valve as in claim 1, wherein the heat conductive layer is made of at least one of aluminum nitride and chrome nitride.
 3. The engine valve as in claim 1, wherein the engine valve is a solid exhaust valve.
 4. The engine valve as in claim 3, wherein the heat insulation layer is formed on at least a back side surface of the head portion.
 5. The engine valve as in claim 1, wherein the heat insulation layer is formed on at least a front side surface of the head portion.
 6. The engine valve as in claim 4, wherein the heat insulation layer is formed on at least a front side surface of the head portion.
 7. The engine valve as in claim 1, wherein the heat insulation layer is made of ceramic-based material.
 8. The engine valve as in claim 2, wherein the heat insulation layer is made of ceramic-based material.
 9. The engine valve as in claim 7, wherein the heat insulation layer is made of at least one of oxide ceramic and nitride ceramic.
 10. An engine valve for mounting to a cylinder head of an engine, comprising: a body having a stem portion and a head portion disposed at one end of the stem portion; a heat insulation layer formed on a surface of the head portion and having a heat conductivity lower than a heat conductivity of the body; and a heat conductive layer formed on a surface on the stem portion and having a heat conductivity higher than the heat conductivity of the body.
 11. The engine valve as in claim 10, wherein: the heat insulation layer is formed on the surface of the head portion that is not in contact with the cylinder head.
 12. The engine valve as in claim 10, wherein the heat conductive layer is formed on at least a part of the surface of the stem portion that can contact with the cylinder head.
 13. The engine valve as in claim 11, wherein the heat conductive layer is formed on at least a part of the surface of the stem portion that can contact with the cylinder head.
 14. The engine valve as in claim 10, wherein the heat conductive layer is made of at least one of aluminum nitride and chrome nitride.
 15. The engine valve as in claim 10, wherein the heat insulation layer is made of at least one of oxide ceramic and nitride ceramic. 